In its simplest form the principle says this: a player should be assumed not to have had a choice, rather than to have exercised a choice in a particular way. The situation in which the principle is most often applied is something like this:
THE most remarkable play I have ever seen or heard of was brought off in the following hand, and the beauty of it was that the play was made without a moment's hesitation, as if there were nothing else to be done.
